GtkIconFactory::add

This method adds a specified icon_set to the icon
factory under the name stock_id. The stock id you
specify should include the name of your application. Normally applications
create a GtkIconFactory and add it to the list of
default factories with add_default() . Then they pass the stock id
to widgets like GtkImage to display the icon.

Themes can provide an icon with the same name to override your
application's default icons. If an icon already existed in the icon factory
for stock_id, it is unreferenced and replaced with the new icon_set.